The invention relates generally to multiple-output, off-line power converters, and more particularly to multiple-output, single winding, bi-directional flyback converters.
There are increasing demands in the electronic industry for power supplies that provide multiple regulated outputs. With various integrated circuits and electronic devices operating at different voltages, the ability to supply efficiently regulated multiple voltage outputs from a single power supply has become increasingly more important.
Flyback converters constitute an economically advantageous solution to implement off-line power supplies with multiple outputs. The principle of bi-directional flyback is well known. Bi-directional flyback was first used in uninterrupted power supplies (UPS) where a bi-directional flyback would charge a battery most of the time, i.e., energy flowing into the battery. When the input power source is interrupted, the energy then flows out of the battery and into the load.
FIG. 1 illustrates a known flyback converter. The flyback converter is comprised of a power supply 102, a transformer 103 with a primary winding 104 and a plurality of secondary windings 106, 112, 114, one for each voltage output, a power supply controller 108, an input switch 110 and post regulators 116, 118. In this example, the flyback implementation uses a single switch 110 on the primary side of the transformer 103 to directly regulate the output voltage VA. The cross-regulation of the rest of the output voltages is achieved by the turns ratio of the secondary windings 112 and 114. However, the presence of leakage inductance and other parasitics results in wide tolerances for the cross-regulated outputs, which do not satisfy strict regulation requirements. The post regulators 116 and 118 are thus necessary to improve the regulation of the output voltages. However, this approach increases the cost and reduces the efficiency of the power supply.
Recently secondary-side control methods have been proposed in European Patent Nos. 0 698 959 and 0 772 284 and U.S. Pat. No. 5,617,015, which include switches on the secondary side of the transformer. The switches on the secondary side of the transformer are used to provide independently regulated outputs, which operate over universal mains voltage and wide load variations. The value of each voltage output can be independently set at different levels by the controller without modification of the circuit.
In European Patent No. 0 698 959, as illustrated in FIG. 2, a separate secondary-side winding 210, 212, 214 is used to generate each output voltage V1, V2, V3, respectively. A rectifying diode 216, 218, 220 is connected in series with each secondary winding 210, 212, 214, respectively. A semiconductor power switch 222, 224 is connected in series with all but the first secondary winding 210. A regulator RS1 of the first output circuit controls the regulator RS0 of the primary side circuit 202. Output regulators RS2 and RS3 control the secondary side switches 222 and 224, respectively. The input dc voltage VIN, a voltage representation of the primary side switch 204 and the output of one of the regulators are also fed to the primary side regulator RS0.
European Patent No. 0 772 284 describes a different secondary-side control method where one secondary winding supplies multiple output voltages via separate branches as illustrated in FIG. 3. Each output voltage V1, V2, V3 is rectified by a diode 302, 304, 306. The first output V1 is regulated by the pulse width of the primary-side switch 308. A switch 310, 312, is inserted in series into each branch supplying the rest of the output voltages V2, V3. A regulator 314 and 316 senses each of the output voltages V2, V3 and controls the duty cycle of the corresponding switch to regulate the output voltage.
U.S. Pat. No. 5,617,015 describes a voltage regulator providing multiple independently regulated outputs as illustrated in FIG. 4. A dedicated switch 402, 404, 406 for each output voltage V1, V2, V3 controls the energy delivered to the output. Energy is delivered to the outputs that have fallen below the lower limit of an acceptable range. A voltage regulator 408 stops supplying the outputs that have exceeded the upper limit of the acceptable range. The input switch 410 directly regulates the output connected to the output switch 402, 404, 406 which is turned on last in the switching sequence.
As noted above, the above-described methods control the on and off state of the primary-side switch based on the values of the output voltages. However, these methods result in hard switching of the primary-side switch which increases the switching losses and reduces the efficiency of the power converter. Thus, there is a need for a bi-directional flyback converter which reduces switching losses by preventing hard switching of the primary-side switch.
It is an object of the invention to overcome the above-described deficiencies of the known flyback converters by providing a multiple-output, bi-directional flyback converter with soft switching of the input switch.
According to one embodiment of the invention, a bi-directional flyback converter is disclosed. The primary-side of the converter is comprised of an input power supply, a transformer with a primary winding and a secondary winding, and an input switch with an associated diode, wherein a first end of the primary winding is connected to the input power supply and a second end of the primary winding is connected to the input switch. A first predetermined number of output voltage circuits are connected to the secondary winding each producing an output voltage. Each output voltage circuit has an output switch with an associated diode, and a diode connected in series between the output voltage switch and the secondary winding. A last output voltage circuit with a bi-directional switch and associated diode is connected connection to the secondary winding. A control unit controls the switching of the input switch, the predetermined number of output switches and the bi-directional switch, wherein the switching of the bi-directional switch is controlled so as to allow for soft switching of the input switch.
According to another embodiment of the invention, a bi-directional flyback converter is disclosed. The primary-side of the converter is comprised of an input power supply, a transformer with a primary winding and a secondary winding, and an input switch with an associated diode, wherein a first end of the primary winding is connected to the input power supply and a second end of the primary winding is connected to the input switch. A first predetermined number of output voltage circuits are connected to a separate secondary winding each producing an output voltage. Each output voltage circuit has an output switch with an associated diode, and a diode connected in series between the output voltage switch and the secondary winding. A last output voltage circuit with a bi-directional switch and associated diode is connected connection to a separate secondary winding. A control unit controls the switching of the input switch, the predetermined number of output switches and the bi-directional switch, wherein the switching of the bi-directional switch is controlled so as to allow for soft switching of the input switch.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereafter.